1. Field of the Invention
The present invention relates to a communication/broadcasting system.
2. Description of the Related Art
In a communication/broadcasting system, a link performance may be considerably deteriorated by various noises of a channel and a fading phenomenon and an Inter-Symbol Interference (ISI). Therefore, to realize high speed digital communication/broadcasting systems requiring a high amount of data processing and high reliability such as next generation mobile communication, digital broadcasting, and a mobile Internet, development of a technology for overcoming a noise and fading and ISI is required. Recently, as a part of a study for overcoming a noise, etc., a study on an error-correcting code is in active progress as a method for raising reliability of communication and broadcasting by efficiently recovering distortion of information.
A Low Density Parity Check (LDPC) code first introduced in 1960s by Gallager has been forgotten due to complexity difficult to realize at the technology level of that time. However, as a turbo code suggested by Berrou and Glavieux, Thitimajshima in 1993 showed performance approaching a channel capacity of Shannon, various analyses for the performance and characteristic of the turbo code have been made and various studies on iterative decoding and channel coding based on a graph have been carried out. With this as a momentum, the LDPC code has been re-studied in the late 1990s and it has been revealed that when decoding is performed with application of iterative decoding based on a sum-product algorithm on a Tanner graph corresponding to the LDPC code, performance approaching a channel capacity of Shannon is obtained.
The LDPC code is defined as a parity-check matrix generally, and may be expressed using a bipartite graph commonly called as a Tanner graph. An LDPC encoder receives an LDPC information word consisting of Kldpc bits to generate an LDPC codeword consisting of Nldpc bits. Hereinafter, for convenience in description, it is assumed that an information word including Kldpc bits is received, so that a codeword consisting of Nldpc bits is generated. That is, when I=[i0, i1, i2, . . . , iKldpc−1] is LDPC-encoded, an LDPC codeword c=[c0, c1, c2, c3, . . . , cNldpc−1] is generated. That is, the LDPC codeword is a bit string consisting of a plurality of bits, and an LDPC codeword bit means respective bits. Also, the LDPC information word is a bit string consisting of a plurality of bits, and an information bit means each bit forming an information word. At this point, in case of a systematic code, an LDPC codeword c=[c0, c1, c2, c3, . . . , cNldpc−1]=[i0, i1, . . . , iKddpc−1, p0, p1, . . . , pNparity−1] is configured. Here, P=[p0, p1, . . . , pNparity−1] is parity bits, and the number of parity bits is Nparity=Nldpc−Kldpc.
The LDPC encoding includes a process for determining a codeword meeting a condition of Equation (1).H·cT=0 where c=[c0,c1,c2, . . . ,cNldpc−1]  (1)
In Equation (1), H is a parity test matrix, c is a codeword, ci is an i-th bit of a codeword, and Nldpc is a codeword length.
The parity test matrix H consists of Nldpc columns, and an i-th column means it is related to an i-th codeword bit ci.
Generally, according to an LDPC code, encoding is performed under a situation where an information word length and a codeword length have been determined as Kldpc and Nldpc in advance. Therefore, in the case where an information word of a length shorter than Kldpc is input or in case of generating a codeword of a length shorter than Nldpc, an appropriate method is required. For example, in the case where an information word having a length of Kl is input to an encoder, a transmission end shortens Kldpc−Kl bits. Kl is shorter than the length Kldpc of an information word required by an encoder. Also, in the case where the length Ntxparity of a required parity is smaller than the parity length Nparity, a transmission end punctures Nparity−Ntxparity bits. Nparity is the length of an actually transmitted parity, and is determined based on Kl and an encoding rate required for transmission.
In the case where a portion of a bit is shortened or punctured with consideration of an information word and the length of a parity, the performance of a codeword may change remarkably based on which bit is shortened or punctured. Therefore, a method for selecting the shortened bit and punctured bits so that an optimized performance is maintained should be proposed.